Strain gage devices have been used for many years to measure strain induced in a structure by application of some deforming force. Older gases comprised one or more thin wires or metal foils which were glued to the structure to be tested, usually with an electrically insulating substrate such as paper between the gage wire or foil and the underlying structure. Strains in the structure would be transmitted to the gage wire, thus modifying its electrical properties in a manner proportional to the applied force. More modern gages comprise thin film resistance elements deposited directly on the deformable structure using sputtering, vacuum evaporating or other thin film producing techniques.
Whatever the type of gage used, those skilled in the art have long recognized that the performance of many gages is highly temperature dependent. For example, the zero setting of the gage may shift due to changes in ambient temperature and temperature gradients in the underlying support structure thus producing an apparent strain when no load has been applied. Also, the span of the gage may change with temperature so that a given change in output of the Wheatstone bridge commonly used in such applications may indicate different applied forces depending on the temperature at which the gage is operating. Numerous attempts have been made to compensate strain gage transducers for the effects of temperature, in the hope of producing a substantially temperature independent transducer.
Ruge discloses in U.S. Pat. Nos. 2,350,972 and 2,390,038 types of strain gages in which one gage wire is chosen to have a negative thermal coefficient of resistance (TCR); and the other, to have a positive TCR. Ruge's U.S. Pat. No. 2,672,048 discloses a temperature compensated strain gage device in which the resistances of the gage elements in a bridge are adjusted in proportion to their thermal coefficients, thus reducing thermal effects. Gay's U.S. Pat. No. 3,184,962 discusses changes in the characteristics of the underlying support structure with temperature; but Gay appears to ignore any thermally induced variation in the gage factor.
U.S. Pat. No. 3,196,668 issued to McLellan discloses a semiconductor strain gage in which the resistivities of gage elements of opposite conductivity types are carefully matched to provide temperature compensation of zero balance and the temperature coefficients of the gage elements are chosen to be identical and positive to give identical resistance changes with temperature. Russell disclosed in U.S. Pat. Nos. 3,245,016 and 3,448,607 types of temperature compensated wire strain gages in which temperature coefficients of resistance and expansion were considered to reduce temperature sensitivity in wire gages. In U.S. Pat. No. 3,290,928, Curry recognized the existence of changes in gage output due to temperature differential among the gages and due to thermally induced strain in the underlying structure and used geometric placement of the gages and external circuitry to provide compensation. Stedman's U.S. Pat. No. 3,303,693 shows an arm type film bridge in which compensation is provided by a compensating resistance in series with certain gage resistances, the compensating resistance having an opposite thermal coefficient of resistance.
Watanabe et al disclose in U.S. Pat. No. 3,609,625 a type of semiconductor strain gage in which the TCR of the gage material is essentially constant over a range of temperatures. More recently, Ort shows in U.S. Pat. No. 4,116,075 a technique for minimizing thermal imbalances between resistances deposited on a flexure element, by providing asymmetric gage geometries.